Forcing Flower Bulbs & Growing Pre-chilled Bulbs

You've had fun with forcing no-chill amaryllis and paperwhite bulbs and now are ready to experiment with other bulbs? Great! Here's some background information to help understand the basics.

The Basic Process

Many types of flower bulbs require a winter dormancy period to bloom and thrive. In their native lands, this happens naturally with late autumn temperature drops, progressively shorter days and eventually, winter's cold. With forcing - we need a better term that doesn't sound so violent - the bulbs are coaxed into believing that all is normal, and as long as their basic needs are met, they're none the wiser.  

First: The Chill

Tulips, hyacinths, most daffodils, crocuses, grape hyacinths, scilla, many bulbous irises and other classic Dutch bulbs require an extended period of cooling to trigger the biochemical processes that result in flowering. Simply put: no chill, no blooms. The ideal temperature range is between 35 and 42 degrees F. The required chill period varies by type of bulb. Here are guidelines from the Neatherland Flower Bulb Information Center.

  • Crocuses - 14-15 weeks
  • Daffodils (narcissus) - 15-17 weeks 
  • Grape Hyacinths - 14-15 weeks   
  • Hyacinths -  11-14 weeks 
  • Irises (reticulata and other bulb types) - 12-14 weeks
  • Tulips - 14-18 weeks 
Conventional wisdom holds that hyacinths, grape hyacinths and compact daffodils are the easiest Dutch bulbs to force. How adventurous are you feeling?

Second: The Rooting-In

Your bulbs must develop strong roots to support the soon-to-bloom plants. Roots anchor the plant and allow the plant to absorb the moisture needed for growth.

The bulbs can be planted and chilled, or can be pre-chilled and then planted. Pre-chilled bulbs need several weeks of dark and cool temperatures (35-45 degrees) after being potted up to grow the required root network. 

Planting the Bulbs

The easiest way to force bulbs is to combine the two steps above by planting in containers from late September to early November and tucking the pots in a chilly spot for the required number of weeks. Suitable sites include garages, basements, crawlspaces and enclosed porches that maintain the desired temperature range. Keep the bulbs in the dark or they may be encouraged to start developing top growth before they are fully chilled. 

A potting mix of 4 scoops of good soil plus 1 scoop of sand creates a blend that drains nicely and discourages mold. Choose containers that provide 1.5-2" of root space below the base of the bulbs. Fill your pot with an inch of gravel or small stones and add your soil mixture. Position the bulbs so small ones - crocuses, grape hyacinths and irises - are covered by 1" of soil. Tulip, daffodil and hyacinth noses can poke a little above the soil. Water well.  

If you are planting pre-chilled bulbs, they have already had their "winter" but they still need three to four weeks of cool temperatures to develop roots. Plant, water well and place in a cool, dark location.

For pre-chilled bulbs that are planted outdoors, best results are achieved in regions where temperatures are moderate to cool. Pre-chilled bulb planted outside in areas where spring comes early, and temperatures move into the lows 50s and warmer, may not receive the cooling period needed for root growth. 

While your bulbs are chilling and growing roots, keep the soil lightly moist. 

Are They Ready?

A good indication that the bulbs are ready is the appearance of roots at the drainage hole on the bottom of the container. When roots appear, relocate the pots into a cool room to encourage the plants begin to develop sprouts. Is your container too large to check the bottom holes? Gently wiggle the noses of a few bulbs - do they feel tightly anchored and well rooted?

As the top growth develops the pots can be moved into a warmer area. A room with temperatures in the lows 60s is ideal.

Note: Moving the freshly rooted bulbs directly into a warm or hot room often results in plants that flower before they reach their normal height, i.e. they are stunted. Remember, we're trying to fool the bulbs into believing that they're outdoors in the cool spring air.

Tips

  1. Flower bulbs can be chilled in a refrigerator where no fruits or vegetables are being held. Many types of ripening produce give off ethylene gas which can damage or kill flower bulb sprouts. Flower bulbs and apples/avacados/tomatoes/etc. don't mix well.
  2. Bulbs in containers are more cold sensitive than bulbs planted in the ground. If you're planting outdoors, monitor temperatures and don't allow your forcing bulbs to freeze.
  3. Planting pre-chilled bulbs outdoors in very warm regions (TX, AZ, FL, NV, toasty CA regions, etc.) often results in insufficient root growth due to the lack of cool, rooting-in temperatures. Without strong roots, the bulbs will struggle to grow and bloom.